The parametric stereo coding technique is the optimal sound compressing technique for mobile devices, broadcasting and the Internet, as it significantly improves the efficiency of a codec for a low bit rate stereo signal, and has been adopted for High-Efficiency Advanced Audio Coding version 2 (Hereinafter, referred to as “HE-AAC v2”) that is one of the standards adopted for MPEG-4 Audio.
FIG. 15 illustrates a model of stereo recording. FIG. 15 is a model of a case in which a sound emitted from a given sound source x(t) is recorded by means of two microphones 1501 (#1 and #2).
Here, C1x(t) is a direct wave arriving at the microphone 1501 (#1), and c2h(t)*x(t) is a reflected wave arriving at the microphone 1501 (#1) after being reflected on a wall of a room and the like, t being the time and h(t) being an impulse response that represents the transmission characteristics of the room. In addition, the symbol “*” represents a convolution operation, and c1 and c2 represent the gain. In the same manner, c3x(t) is a direct wave arriving at the microphone 1501 (#2), and c4h(t)*x(t) is a reflected wave arriving at the microphone 1501 (#2). Therefore, assuming signals recorded by the microphones 1501 (#1) and (#2) as l(t) and r(t), respectively, l(t) and r(t) can be expressed as the linear sum of the direct wave and the reflected wave as in the following equations.l(t)=c1x(t)+c2h(t)*x(t)  [Equation 1]r(t)=c3x(t)+c4h(t)*x(t)  [Equation 2]
Since an HE-AAC v2 decoder cannot obtain a signal corresponding to the sound source x(t) in FIG. 15, a stereo signal is generated approximately from a monaural signal s(t), as in the following equation. In Equation 3 and Equation 4, each first term approximates the direct wave and each second term approximates the reflected wave (reverberation component).l′(t)=c1′s(t)+c2′h(t)*s(t)  [Equation 3]r′(t)=c3′s(t)+c4′h═(t)*s(t)  [Equation 4]
While there are various methods for generating a reverberant component, a parametric stereo (hereinafter, may be abbreviated as “PS” as needed) decoding unit in accordance with the HE-AAC v2 standard generates a reverberation component d(t) by decorrelating (orthogonalizing) a monaural signal s(t), and generates a stereo signal in accordance with the following equations.l′(t)=c1′s(t)+c2′d(t)  [Equation 5]r′(t)=c3′s(t)+c4′d(t)  [Equation 6]
While the process has been explained as performed in the time region for explanatory purpose, the PS decoding unit performs the conversion to pseudo-stereo in a time-frequency region (Quadrature Mirror Filterbank (QMF) coefficient region), so Equation 5 and Equation 6 are expressed as follows, where b is an index representing the frequency, and t is an index representing the time.l′(b,t)=h11s(b,t)+h12d(b,t)  [Equation 7]r′(b,t)=h21s(b,t)+h22d(b,t)  [Equation 8]
Next, a method for generating a reverberation component d(b,t) from a monaural signal s(b,t) is described. While there are various method for generating a reverberation component, the PS decode unit in accordance with the HE-AAC v2 standard converts the monaural signal s(b,t) into the reverberation component d(b,t) by decorrelating (orthogonalizing) it using an IIR (Infinite Impulse Response)-type all-pass filter, as illustrated in FIG. 16.
The relationship between input signals (L, R), a monaural signal s and a reverberation component d is illustrated in FIG. 17. As illustrated in FIG. 17, the angle between the input signals L, R and the monaural signal s is assumed as α, and the degree of similarity is defined as cos(2α). An encoder in accordance with the HE-AAC v2 standard encodes α as the similarity information. The similarity information represents the similarity between the L-channel input signal and the R-channel input signal.
FIG. 17 illustrates, for the sake of simplification, an example of a case in which the lengths of L and R are the same. However, in consideration of a case in which the lengths (norms) of L and R are different, the ratio of the norms of L and R is defined as an intensity difference, and the encoder encodes it as the intensity difference information. The intensity difference information represents the power ratio of the L channel input signal and the R channel input signal.
A method for generating a stereo signal from s(b,t) and d(b,t) at the decoder side is described. In FIG. 18, S is a decoded input signal, D is a reverberation signal obtained at the decoder side, CL is a scale factor of the L channel signal calculated from the intensity difference. A vector obtained by combining the result of the projection, in the direction of the angle α, of the monaural signal that has been subjected to scaling using CL, and the result of the projection, in the direction of (π/2)−α, of the reverberant signal that has been subjected to scaling using CL is regarded as the decoded L channel signal, which is expressed as Equation 9. In the same manner, the R channel may also be generated in accordance with Equation 10 below using the scale factor CR, S, D and the angle α. There is a relationship CL+CR=2 between CL and CR.
                                                                                          L                  ′                                ⁡                                  (                                      b                    ,                    t                                    )                                            =                                                                    C                    L                                    ⁢                                      s                    ⁡                                          (                                              b                        ,                        t                                            )                                                        ⁢                  cos                  ⁢                                                                          ⁢                  α                                +                                                      C                    L                                    ⁢                                      d                    ⁡                                          (                                              b                        ,                        t                                            )                                                        ⁢                                      cos                    ⁡                                          (                                                                        π                          2                                                -                        α                                            )                                                                                                                                              =                                                                    C                    L                                    ⁢                                      s                    ⁡                                          (                                              b                        ,                        t                                            )                                                        ⁢                  cos                  ⁢                                                                          ⁢                  α                                +                                                      C                    L                                    ⁢                                      d                    ⁡                                          (                                              b                        ,                        t                                            )                                                        ⁢                  sin                  ⁢                                                                          ⁢                  α                                                                                        [                  Equation          ⁢                                          ⁢          9                ]                                                                                                      R                  ′                                ⁡                                  (                                      b                    ,                    t                                    )                                            =                                                                    C                    R                                    ⁢                                      s                    ⁡                                          (                                              b                        ,                        t                                            )                                                        ⁢                                      cos                    ⁡                                          (                                              -                        α                                            )                                                                      -                                                      C                    R                                    ⁢                                      d                    ⁡                                          (                                              b                        ,                        t                                            )                                                        ⁢                                      cos                    ⁡                                          (                                                                        π                          2                                                -                        α                                            )                                                                                                                                              =                                                                    C                    R                                    ⁢                                      s                    ⁡                                          (                                              b                        ,                        t                                            )                                                        ⁢                                      cos                    ⁡                                          (                                              -                        α                                            )                                                                      +                                                      C                    R                                    ⁢                                      d                    ⁡                                          (                                              b                        ,                        t                                            )                                                        ⁢                                      sin                    ⁡                                          (                                              -                        α                                            )                                                                                                                              [                  Equation          ⁢                                          ⁢          10                ]            
Therefore, Equation 9 and Equation 10 can be put together as Equation 11.
                                          [                                                                                                      L                      ′                                        ⁡                                          (                                              b                        ,                        t                                            )                                                                                                                                                              R                      ′                                        ⁡                                          (                                              b                        ,                        t                                            )                                                                                            ]                    =                                    [                                                                                          h                      11                                                                                                  h                      12                                                                                                                                  h                      21                                                                                                  h                      22                                                                                  ]                        ⁡                          [                                                                                          s                      ⁡                                              (                                                  b                          ,                          t                                                )                                                                                                                                                        d                      ⁡                                              (                                                  b                          ,                          t                                                )                                                                                                        ]                                      ⁢                                  ⁢        where        ⁢                                  ⁢                                                                                                  h                    11                                    =                                                            C                      L                                        ⁢                    cos                    ⁢                                                                                  ⁢                    α                                                  ,                                                                                      h                  12                                =                                                      C                    L                                    ⁢                  sin                  ⁢                                                                          ⁢                  α                                                                                                                                              h                    21                                    =                                                            C                      R                                        ⁢                                          cos                      ⁡                                              (                                                  -                          α                                                )                                                                                            ,                                                                                      h                  22                                =                                                      C                    R                                    ⁢                                      sin                    ⁡                                          (                                              -                        α                                            )                                                                                                                              [                  Equation          ⁢                                          ⁢          11                ]            
A conventional example of a parametric stereo decoding apparatus that operates in accordance with the principle described above is explained below.
FIG. 19 is a configuration diagram of a conventional parametric stereo decoding apparatus.
First, a data separation unit 1901 separates received input data into core encoded data and PS data.
A core decoding unit 1902 decodes the core encoded data, and outputs a monaural sound signal S(b), where b is an index of the frequency band. As the core decoding unit, one in accordance with the conventional audio coding/decoding system such as the AAC (Advanced Audio Coding) system and the SBR (Spectral Band Replication) system.
The monaural sound signal S(b) and the PS data are input to a parametric stereo (PS) decoding unit 1903.
The PS decoding unit 1903 converts the monaural signal S(b) into stereo decoded signals L(b) and R(b), on the basis of the information of the PS data.
Frequency-time conversion units 1904(L) and 1904(R) convert the L-channel frequency region decoded signal L(b) and the R-channel frequency region decoding signal R(b) into an L channel time region decoded signal L(t) and an R channel time region decoded signal R(t), respectively.
FIG. 20 is a configuration diagram of the PS decoding unit 1903 in FIG. 19.
In accordance with the principle mentioned in the description of FIG. 16, to the monaural signal S(b), a delay is applied by a delay adder 2001, and decorrelation is performed by a decorrelation unit 2002, to generate the reverberation component D(b).
In addition, a PS analysis unit 2003 analyzes PS data to extract the degree of similarity and the intensity difference. As mentioned above in the description of FIG. 17, the degree of similarity represents the degree of similarity of the L-channel signal and the R-channel signal (which is a value calculated from the L-channel signal and the R-channel signal and quantized, at the encoder side), and the intensity difference represents the power ratio between the L-channel signal and the R-channel signal (which is a value calculated from the L-channel signal and the R-channel signal and quantized in the encoder).
A coefficient calculation unit 2004 calculates a coefficient matrix H from the degree of similarity and the intensity difference, in accordance with Equation 11 mentioned above.
A stereo signal generation unit 2005 generates stereo signals L(b) and R(b) on the basis of the monaural signal S(b), the reverberation component D(b) and the coefficient matrix H, in accordance with Equation 12 below that is equivalent to Equation 11 described above.L(b)=h11S(b)+h12D(b)R(b)=h21S(b)+h22D(b)  [Equation 12]
Studied below is a case in which, in the conventional art of the parametric stereo system described above, stereo signal having little correlation between an L-channel input signal and an R-channel input signal, such as a two-language sound is encoded.
Since the stereo signal is generated from a monaural signal S at the decoder side in the parametric stereo system, the characteristics of the monaural signal S have influence on output signals L′ and R′, as can be understood from Equation 12 mentioned above.
For example, when the original L-channel input signal and R-channel signal are completely different (i.e., the degree of similarity is zero), the output sound from the PS decoding unit 1903 in FIG. 19 is calculated in accordance with the following equation.L′(b)=h11S(b)R′(b)=h21S(b)  [Equation 13]
The component of the monaural signal S appears in the output signals L′ and R′, which is schematically illustrated in FIG. 21. Since the monaural signal S is the sum of the L-channel input signal and the R-channel input signal, Equation 13 indicates that one signal leaks in the other channel.
For this reason, in the conventional parametric stereo decoding apparatus, there has been a problem that when listening output signals L′ and R′ at the same time, similar sounds are generated from left and right, creating an echo-like sound and leading to the deterioration of the sound quality.
[Patent document 1]: Japanese Laid-open Patent Application No. 2007-79483